The invention relates to a method for determining variations in the position of a movable object, comprising the steps of:
1) generating clock pulses at a predetermined clock frequency F.sub.CL and a sequence of transmission start signals having a repetition frequency F.sub.R and transmitting a burst of ultrasonic waves having a frequency F.sub.US towards the object each time a transmission start signal has been generated;
2) starting counting clock pulses at a time that has a predetermined relationship with the time when a start signal is generated;
3) receiving ultrasonic waves having the frequency F.sub.US reflected by the object and forming an analog echo signal that represents the electrical equivalent of the received ultrasonic waves;
4) converting the analog echo signal into a digital echo signal comprising a sequence of echo pulses at a repetition frequency F.sub.US, each echo pulse representing a period of the ultrasonic waves;
5) generating a time-window that has a predetermined width;
6) determining whether a predetermined edge of an echo pulse occurs within the time-window;
7) setting the time for generating the next time-window at 1/F.sub.R -1/2F.sub.US seconds after an echo pulse has occurred within the time-window or at an estimated value if no echo pulse has occurred within the time-window;
8) stopping the counting of clock pulses on the occurrence of a predetermined edge of an echo pulse in the time-window.
The invention also relates to a device for determining variations in the position of a movable object, comprising:
1) a timer for generating clock pulses at a predetermined clock frequency F.sub.CL, a sequence of transmission start signals having a repetition frequency F.sub.R and reset signals having the same repetition frequency F.sub.R and a predetermined phase relationship with the transmission start signals, the clock pulses, transmission start signals and reset signals being made available on first, second and third outputs, respectively, of the timer;
2) transmitter means having its input connected to the second output of the timer for transmitting a burst of ultrasonic waves having a frequency R.sub.US towards the object upon reception of a transmission start signal;
3) receiver means for receiving ultrasonic waves having the frequency F.sub.US reflected by the object and making available on its output an analog echo signal that represents the electrical equivalent of the received ultrasonic waves;
4) converter means for converting the analog echo signal into a digital echo signal comprising a sequence of echo pulses having a repetition frequency F.sub.US, each echo pulse representing a period of the ultrasonic waves, the converter means having an input connected to the output of the receiver means and making available the digital echo signal on its output;
5) control means for generating a time-window that has a predetermined width, determining whether a predetermined edge of an echo pulse occurs within the time-window, setting the time for generating the next time-window at 1/F.sub.R -1/2F.sub.US seconds after an echo pulse has occurred within the time-window or at an estimated value if no echo pulse has occurred within the time-window and generating a counter stop signal on the occurrence of a predetermined edge of an echo pulse in the time-window, the control means having a first input connected to the output of the converter means and an output on which the counter stop signal is made available;
6) counter means having a first input connected to the first output of the timer, a second input connected to the third output of the timer and a third input connected to the output of the control means.
A method and a device of this kind are known from EP-A-0 356 629. Such a method can be used in medicine for measuring the movements of a boundary between different tissues, e.g. the movements of a wall of an artery. It can also be used for detecting the movements of a person's chest in order to serve as a respiration sensing means.
The known method has the disadvantage that a manual start is necessary if no echo pulse has been received in the previous time-window. This means that the operator must manually activate the control means each time an echo pulse has arrived outside the time-window. If the operator fails to restart the measuring sequence without loosing too much time a serious loss of information could be the result. This is very inconvenient and can even be dangerous, e.g. when the device is used as a respiration sensor.